(a) Experimental set-up: the MFM tip is mechanically driven by a piezoelectric transducer near its resonant frequency and kept at a constant height above the surface. The top electrode and the MFM tip are both grounded, and a DC bias is directly applied to the interface.^[135] (b) Kelvin-probe force microscopy measurement of a region that includes the area for which MFM measurements are made.^[134] The top gate is grounded, and voltage bias to the interface (V_dc) is 3 V. The topography is shown as height, while the colour maps onto the measured surface potential (the work function is already subtracted). (c) MFM frequency images over a 3 mm×3 mm area indicated by the black dashed line-enclosed region in panels (b). The MFM tip is magnetized horizontally parallel to the [010] sample direction.^[135] MFM frequency images for V_dc increasing from −4 V to 0 V then decreasing to −4 V. Magnetic domain features are clearly observed for V_dc < − 2 V. The final state 2, obtained after cycling the voltage to V_dc = 0, is uncorrelated with the initial state 1. The regions enclosed by dashed lines give examples of where magnetic contrast is unchanged (region 1) or reversed (region 2) after voltage cycling.